*Please note: you may not see animations, interactions or images that are potentially on this page because you have not allowed Flash to run on S-cool. To do this, click here.*

Life of Stars

Scientists worked out a fairly long time ago that the Sun couldn't be powered by chemical reactions because it would have used up all its energy long before life was formed. It wasn't until nuclear reactions were understood that they could work out what was going on inside the Sun.

Stars start off in the same way, but their end depends on how much mass they have.

Step 1:

Gravitational attraction pulls hydrogen atoms together. The initial star will be big and cold. As it pulls its mass closer together, the hydrogen atoms will start to fuse together to make helium atoms. This is called nuclear fusion. A huge amount of energy is released during nuclear fusion, compared with a chemical reaction. (Don't confuse this reaction with nuclear fission, which they use to generate electricity.)

The picture below shows a birth place for stars:

Step 2:

The star will settle down into middle age. The outward pressure, caused by the highly energetic nuclear reactions taking place in the star, just cancels out the gravitational attraction pulling the star in on itself. A star spends most of its lifetime like this. Our sun is at this stage in its development.

Step 3:

Eventually, the hydrogen in the star is used up. When this happens, the star starts to collapse under its own gravitational attraction.

There is no longer enough outward pressure from nuclear reactions to stop the star collapsing, this causes the star to become unstable.

A sudden surge of radiation is emitted, which causes the star to expand massively.

This will happen to our Sun in about 5 billion years time - it will then expand to the orbit of Mars.

The animation below shows this process:

The Star is now a red giant. It will stay like this for a long time with helium nuclei joining together to make heavier elements.

These stars have shorter life spans than smaller stars. They become red giants very quickly (in fact, they are called red super giants because they are so big).

When they explode, they cause a supernova:

Image credit: NASA

The collapsed core will either form a neutron star - where the gravitational attraction has pulled everything together so much all the particles have turned in to neutrons, or a black hole - which is so dense, even light can not escape from its gravitational field. Neutron stars are formed if the star's mass is between 1½ and 3 times the mass of our sun. A black hole is formed if the star's mass is over 3 times the mass of our Sun.